.\"	$OpenBSD: vfork.2,v 1.19 2015/03/21 22:22:03 jmc Exp $
.\"	$NetBSD: vfork.2,v 1.8 1997/07/10 07:54:13 mikel Exp $
.\"
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.\"     @(#)vfork.2	8.1 (Berkeley) 6/4/93
.\"
.Dd $Mdocdate: March 21 2015 $
.Dt VFORK 2
.Os
.Sh NAME
.Nm vfork
.Nd spawn new process and block parent
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft pid_t
.Fn vfork void
.Sh DESCRIPTION
.Fn vfork
was originally used to create new processes without fully copying the address
space of the old process, which is horrendously inefficient in a paged
environment.
It was useful when the purpose of
.Xr fork 2
would have been to create a new system context for an
.Xr execve 2 .
Since
.Xr fork 2
is now efficient, even in the above case, the need for
.Fn vfork
has diminished.
.Fn vfork
differs from
.Xr fork 2
in that the parent is suspended until the child makes a call to
.Xr execve 2
or an exit (either by a call to
.Xr _exit 2
or abnormally).
In addition, fork handlers established using
.Xr pthread_atfork 3
are not called when a multithreaded program calls
.Fn vfork .
.Pp
.Fn vfork
returns 0 in the child's context and (later) the PID of the child in
the parent's context.
.Sh RETURN VALUES
Same as for
.Xr fork 2 .
.Sh SEE ALSO
.Xr execve 2 ,
.Xr fork 2 ,
.Xr sigaction 2 ,
.Xr wait 2
.Sh HISTORY
The
.Fn vfork
function call appeared in
.Bx 3.0
with the additional semantics that the child process ran in the
memory of the parent until it called
.Xr execve 2
or exited.
That sharing of memory was removed in
.Bx 4.4 ,
leaving just the semantics of blocking the parent until the child calls
.Xr execve 2
or exits.
On many other systems the original behavior has been restored,
making this interface particularly unportable.
.Sh BUGS
To avoid a possible deadlock situation, processes that are children
in the middle of a
.Fn vfork
are never sent
.Dv SIGTTOU
or
.Dv SIGTTIN
signals; rather, output or
.Xr ioctl 2
calls are allowed and input attempts result in an end-of-file indication.
